1. Field of the Invention
This invention relates to a skew jump correcting circuit for providing sharp and clear pictures free from flickering.
2. Background
In recording video signals on magnetic tapes, magnetic disks or other recording media, in general signals of one frame are alloted to one track or the signals of one field are alloted to one track. The 1-field/1-track recording system includes a 1-frame/2-track recording system in which odd-number fields and even-number fields are alternately recorded, and a field recording system in which only odd-number fields or only even-number fields are recorded.
In reproduction according to the 1-frame/2-track recording system, odd-number field signals F1 and even number field signals F2 are successively reproduced from the track as shown in the part (a) of FIG. 5 and are used alternately for the field scanning of the screen of the television set, so that frame scanning is carried out in an interlaced scanning mode. On the other hand, in reproduction according to the field recording system, vertical correlation of video signals is utilized, and one kind of signal produced repeatedly from one and the same track, such as an odd-number field signal F1 as shown in the part (b) of FIG. 5, is converted into a frame signal, to perform frame scanning.
Accordingly, in the field recording system, recording density is twice as high as that in the 1-frame/2-track recording system, with the result that recording is lengthy for motion pictures and the number of frames can be increased for still pictures.
However, in reproduction according to the field recording system, the following difficulty is encountered when the field signal is converted into the frame signal.
One field consists of 262.5 horizontal scanning periods. Therefore, if field scanning is carried out repeatedly with the same field signal F1, then the horizontal synchronizing signal interval jumps by 0.5 H (H=horizontal scanning period) when the end part of the field signal F1 is followed by the beginning part as indicated by the arrow J in part (b) of FIG. 5. This jump is too large for the horizontal synchronization AFC of the television set to pull in. Therefore, the synchronization becomes irregular, as a result of which the upper portion of the reproduced picture is shifted sidewardly, that is, so-called "skew distortion" occurs with the reproduced picture.
In order to overcome this difficulty, heretofore the following method has been employed. As shown in FIG. 6, the same field signal F1 produced repeatedly from one and the same track is applied to a 0.5 H delay line 1. Every vertical scanning period (IV), the through field signal F3 and the 0.5 H delayed field signal F4 are alternately selected by an analog switch 2, so that the odd-number field signal F3 and the even-number field signal F4, which are similar to the odd-number field signal F1 and the even-number field signal F2 shown in FIG. 5, are combined to provide a frame signal having continuous horizontal synchronizing signal intervals for the interlaced scanning. In this case, the vertical synchronizing signal interval is changed by 0.5 H from 1V. In order to overcome this drawback, the contacts c and d of the analog switch 2 are selected as shown in FIG. 7. That is, of the through field signal F3 selecting period, the part K between the front equalizing pulse interval and the rear equalizing pulse interval is utilized for selection of the 0.5 H delayed field signal F4.
In FIG. 5, reference character S designates a video signal; FE, a front equalizing pulse; V, a vertical synchronizing signal; BE, a back equalizing pulse; N, a slot pulse; and H1 through H525, horizontal synchronizing signals.
The above-described prior art suffers from the following difficulties. Because of the temperature characteristic, transfer efficiency and transfer noise of the 0.5 H delay line 1, the 0.5 H delayed field signal F4 is attenuated, limited in band width or lowered in S/N ratio. Furthermore, as the offset voltages at the contacts c and d of the analog switch 2 are different from each other, in the resultant frame signal the odd-number field signal F3 and the even-number field signal F4 are different especially in level from each other. As a result, the produced picture flickers.
These difficulties occur not only with the reproduction of a still video according to the field recording system, but also with the still picture reproduction according to the recording system for video tape recorders in which a so-called "H arrangement" (that the horizontal synchronizing signals of adjacent tracks are arranged perpendicularly to the tracks) is shifted. That is, in the recording system in which the horizontal synchronizing signals of adjacent tracks are shifted by 0.25 H or 0.75 H from each other, in the reproduction a skew jump of 0.5 H occurs every two fields, resulting in skew distortion. Therefore, similarly as in the case of FIG. 6, a 0.5 H delay line and an analog switch are provided, so that the continuity of the horizontal synchronizing signal intervals is maintained by switching the through reproducing video signal and the 0.5 H delayed reproducing video signal. However, in this case also, the through reproducing video signal and the 0.5 H delayed reproducing video signal are different in level, with the result that the reproduced picture flickers.
In view of the foregoing, an object of this invention is to provide a circuit which, when the field signal is repeatedly reproduced, can correct the discontinuity (skew jump) of the horizontal synchronizing signal intervals without causing the reproduced picture to flicker.